Electrode coating for discharge devices



Nov. 2l, 1950 E, F. LowRY ETAL 2,530,394

ELECTRODE COATING FOR DISCHARGE DEVICES Filed June 5, 1947 Erzanl' luxury 1. Eric L-Hagcr Gerallwlfz IN VEN TOILSr Bydm-M M4;

ATTORNEY Patented Nov. 21, 1950 ELECTRODE COATING FOR DISCHARGE DEVICES Erwin F. Lowry, Lynn, Eric L. Mager, Peabody,

' and Gerald W. Kcillioltz, Lynn, Mass., assignors to Sylvania Electric Products, Inc., salem, Mass., a corporation of Massachusetts Application J une 5', 1947, Serial No. 752,698

20 Claims. (Ci. 176-126) This invention relates to electric gaseous discharge devices and more particularly to fluorescent lamps and the composition of the coating material with which the lamp electrodes are provided.

An object of this invention is to provide an eiiicient electrode coating for electric gaseous discharge devices. i

Another object is to provide an electrode coating which will eifect a lengthening of the life of the device in which the electrode is employed.

A further object is to provide an electrode coating which will enhance the lumens per watt maintenance of the device in which the electrode is employed.

A further oblect is to provide a new coating composition for fluorescent lamp electrodes.

A further object is to provide a coating composition for iluorescent lamp electrodes which will have a. beneficial eiiect in reducing various types and forms of discoloration which sometimes develop on the wall oi the lamp during'life.

Further objects, advantages, and features will be apparent from the following specification when read in conjunction with the accompanying drawing in which the ligure is a view of a nu'orescent lamp partly in section.

In the drawing a light-transmitting tubular envelope I. of glass or the like, is coated on its inner. wall with a iluorescent material 2 such'as tungstates or silicates, for example, magnesium tungstate or zinc beryllium silicate. The envelope 2 is provided, at each end, with a stem 3, through which lead wires 4 and 5 extend.

A tungsten wire lamentary electrode t, coated with electron-emissive material, is mounted on the inner ends of the lead wires 4 and 5. The outer ends of the lead wires 4 and 5 are connected topinslandloiabasewhichissecuredto the ends of the envelope l. The sealed envelope is provided with a illllng of inert gas at a low pressure, such as argon at about 3 mm., and'a small quantity of mercury Il to provide mercury vapor. The lamp is ordinarily operated at a mercury vapor pressure of between 1 to 20 microns.

'In the manufacture of electric gaseous discharge devices, and more particularly in the manufacture of uorescent lamps of the hat cathode" type, i. e., those in which the electrodes comprise a coiled lament oi tungsten wire. such as that''=`shown in U. S. Patent 2,258,158, for example. the electrodes are usually provided with a coating of an electron-emissive material. As in the patent mentioned above, the electrode wire can be wound into a minor coil and the latter itself then wound into a major coil to form a coiled-coil with the electron-emisslve coating material illing the space within the minor coil only. In the case of a fluorescent lamp electrode. the electron emissive material usually coin- 2 prises an oxide oi one or more of the alkaline earth metals. For convenience, the alkaline earth metals are usually applied in the iorm of a carbonate which breaks down to the oxide when the lamp is processed.

Many times, during the life of a iluorescent lamp, discoloration oi' various types and forms appears on the wall oi the lamp near the ends thereof and in proximity to the electrodes. It is an object of this invention to reduce the irequency of occurrence and intensity of these discolorations. Three types of discoloratlons which are aiected by the new electrode coating coinposition of our invention are, electrode spots, end bands, and multiples.

Electrode spots are dark gray or black spots which appear on the inner wall of the lamp near the ends thereof, and are usually found approximately opposite the electrodes. End bands are rings of discoloration, usually light yellow, brownish or black, which usually extend completely around the periphery oi the inner wall of the lamp. End bands are usually deiined by a sharp edge on the side near the electrode and a diliused edge on the side near the longitudinal center of the lamp. End bands are usually round forward of the electrodes with the sharp edge located opposite the end of the positive column discharge. Multiples are narrow crescente of discoloration, usually light yellow, brownish, or black, which appear near the ends of the lamp on both sides of the electrodes, i. e., on the base side of the electrode as well as the side near the longitudinal center of the lamp. l

We have found that the addition of zirconium dioxide in proper amounts to the basic electron emlssive material with which the electrodes are coated will not only reduce the frequency of appearance and intensity of the above described discolorations but will also extend the useful life of the lamp and increase the lumens per watt maintenance during life.

In the preparation of the basic electron emissive material we preier to use a triple carbonate, of the alkaline earth metals, i. e., SrCO:l, BaCOa. and CaCOs. However, a douille or single carbonate may be employed without departing from the spirit of our invention. The carbonates may be prepared in any suitable manner well known in the art. In the preparation of the triple carbonate we have found the following composition to be very satisfactory: about 35% SrCOz, about of ZrOz may vary from between about 1% t0 about 20% by weight based on the total weight oi' the carbonates to which it is added. We have found that the addition o! about 5% by weight based on the weight of the carbonates gives exceptionaliy good results. If less than 1% is used. we have found no appreciable improvement is effected. On the other hand, if more than 20% ZrOz is added, the frequency. of occurrence and intensity of discoloration increases and it becomes more dimcult to start the lamps at normal starting voltages. Tables I, II and III below show the extent to which the addition of small quantities of ZrOz to the electrode coating material effects a reduction in discoloration, an lncrease in the L. P. W. late in life, and a lengtheningof the life of the lamps. The lamps on which these tests were made were exactly the same as those manufactured on standard production runs. with the sole exception that vsome ZrOg was added to the electrode coating material of some of the lamps. The same iiuorescent material, quantity of mercury, amount of gasiling etc., was used.. The lamps were processed on the same exhaust machine at the same time. Other tests have shown results similar to thosetabulated below.

Table I shows, in percent, the number of lamps. of the hundreds which were checked, which were free of any discoloration atl00, 300, and 500 hours of operation. Tests A, B and C represent 40` watt fluorescent lamps in which the electrode coating contained no ZrOz. In Test D, about 5% ZrO-i was added to the velectrode coating material.

' Table 1' Test--. A B o D Hours 100 300 50 100 300 500 Per Cent C l e a n 8016 asas si 3375 s2 439s 90.87

Table IIshows the average L. P. W. of lamps late in life. In this test, ve lamps had no ZrOa in the electrode coating, ilve lamps had 5% ZrOz in the electrode coating and ilve lamps had 10% ZrOz in the electrode coating.

Table II ov zro, 57 zro, 10/ zro'.

L P. w L.1 w.

2544 hours 4o. s 41. e 41. e 4040 hours .8 40.0 40. 4 6131 hours 38.8 37. 8

Table III shows the time at which the lamps of Table II failed.

Table III ZrOz 5% ZrO; 10% ZrOg Haan No failures up to 7,066 hours 989 6, 503 6, 6&5

4 low when compared to the rapid appearance of discoloration at 300 and 500 hours in lamps in which the electrode coating contains no zirconium dioxide. The beneficial eiect which the addition of zirconium dioxide to the electrode coating has on the L. P. W. of the lamps late in life is illustrated in Table lI. For example, lamps with no ZrOz in the electrode coating read 38.8 L. P. W. at 4040 hours whereas this same reading was obtained at 6131 hours with lamps having 5% ZrOz in the electrode coating material. The eiect which the addition of small quantities of ZrOz has on the life as illustrated in Table III, is tremendous. The average life of the lamps with no ZrOz in the electrode coating was about 4200-4300 hours; the first failure of the lamps with ZrOz in the electrode coating did not occur until almost 6000 hours; and the '7000 hour mark has been passed without a single failure among the five lamps with 5% ZrOz in the electrode coating material.

As mentioned above, the tungsten wire illamentary electrodes of uorescent lamps are usually provided with an electron-emissive coating of the oxide of one or more of the alkaline earth metals. For convenience this material is usually applied to the electrodes in the form 'of a carbonate which is later broken down to give the oxide. `For this reason, therefore, the proportions given above with reference to the amount of ZrOz which is added (1% to 20%) are based on the weight of the mixed carbonates to which the ZrO'.` is added. This is equivalent to a percentage range of from slightly less than 1% to about..l7% based upon the total weight of the batch materials after mixing and a percentage range of from slightly more than 1% to about 22% of the total weight of the mixed oxides to which the batch is subsequently converted during lamp processing.. Thefpreierential amount oi.' ZrO: which has been described as about 5% by weight based on the weight of the mixed carbonates is slightly less than 5% by weight of the total batch after mixing and is slightly more than 6% oi' the total weight of the mixed oxides ilnally obtained. The 10% ZrOa in Table III is based on the total weight of the mixed carbonates to which the zirconium dioxide is added. This is equivalent to about 11% of the iinal weight of the mixed oxide, the carbonate being converted to oxides during the lamp processing, as previously explained. y

What we claim is:

1. A 'coating material for electric gaseous discharge device electrodes comprising at least one oxide of an valkaline earth metal and a quantity of zirconium dioxide equal to about 6% by weight of the total weight of all the oxides.

2. A coating material for electric gaseous dis- 60 charge device electrodes comprising oxides of at least two of the alkaline earth metals and a quantity of zirconium dioxide equal to about 6% by weight of the total weight of all the oxides.

3. A coating material for electric gaseous dis- 05 charge device electrodes comprising oxides or tium and calcium and a quantity of zirconium dioxide in an amount more than about 1% but not more than 11% by weight of the total weight of all the oxides.

5. A iluorescent lamp comprising a sealed lighttransmitting envelope, a coating of iiuorescent material on the inner wall of said envelope, a quantity of mercury in said envelope, a lling of inert gas in said envelope, iilamentary electrodes in said envelope, and a coating on said electrodes, said coating comprising at least one oxide of an alkaline earth metal and a quantity of zirconium dioxidel equal to about 6% by weight of the total weight of all the oxides. l

6. A fluorescent lamp comprising a sealed lighttransmitting envelope, a coating of fluorescent material on the inner wall of said envelope, a quantity oi mercury in said envelope, a iilling of inert gas in said envelope, lamentary electrodes in said envelope. and a coating on said electrodes, said coating comprising oxides of at least two of the alkaline earth metals and a quantity of zirconium dioxide equal to about 6% by weight of the total weight of all the oxides.

7. A fluorescent lamp comprising a sealed lighttransmitting envelope, a coating of iiuorescent material on the inner wall of said envelope, a quantity of mercury in said envelope, a iilling of inert gas in said envelope, illamentary electrodes in said envelope, and a coating on said electrodes, said coating comprising oxides of barium, strontium and calcium and a quantity of zirconium dioxide equal to about 6% by weight of the total weight of all the oxides.

8. A fluorescent lamp comprising a sealed lighttransmitting envelope, a coating of fluorescent material on the inner Wall of said envelope, a quantity of mercury in said envelope, a lllng of inert gas in said envelope, fllarnentary electrodes in said envelope, and a coating on said electrodes, said coating comprising at least one oxide of an alkaline earth metal and a quantity of zirconium dioxide in an amount more than about 1% but not more than 11% by weight of the total weight of all the oxides.

9. A fluorescent lamp comprising a sealed lighttransmitting envelope, a coating of iluorescent material on the inner wall of said envelope, a quantity oi' mercury in said envelope, a filling of inert gas in said envelope, iilamentary electrodes in said envelope, and a coating on said electrodes, said coating comprising oxides of at least two of the alkaline earth metals and a quantity of zirconium dioxide in an amount more than about 1% but not more than 11% by weight of the total weight of all the oxides.

10. A iiuorescent lamp comprising a sealed light-transmitting envelope, a coating of fluorescent material on the inner wall of said envelope, a quantity of mercury in said envelope, a filling of inert gas in said envelope, lamentary electrodes in said envelope, and a coating on said electrodes, said coating consisting essentially of at least one oxide of an alkaline earth metal and a quantity of zirconium dioxide in an amount more than about 1% but not more than 11% by weight of the total weight of all the oxides.

11. A iluorescent lamp comprising a sealed light-transmitting envelope, a coating of fluorescent material on the inner wall of said envelope, a quantity of mercury in said envelope, a filling of inert gas in said envelope, fllamentary electrodes in said envelope, and a coating on said electrodes, said coating consisting essentially of oxides of at least two or the alkaline earth metals ,of barium, strontium and calcium and a quantity of zirconium dioxide in an amount more than about 1% but not more than 11% by weight of the total weight of all the oxides.

13. A low-pressure iluorescent lamp comprising an elongated sealed light-transmitting envelope, a coating of uorescent material on the inner wall of said envelope, a quantity of mercury in said envelopes, a fllamentary electrode of coiled tungsten wire at each end of said envelope, and an electron-emissive material carried by said electrode and comprising at least one oxide of an al kaline earth metal and a quantity of zirconium dioxide in an amount between about 1% and 11% by weight of the total weight of all the oxides carried by said electrode.

14. A fluorescent lamp comprising a sealed light-transmitting envelope, a coating of fluorescent material on the inner wall of said envelope, a quantity of mercury of said envelope, a filling of inert gas in said envelope, lamentary electrodes in said envelope, and a coating on said electrodes, said coating comprising atleast one oxide of an alkaline earth metal and a quantity of zirconium dioxide in a critical amount between zero percent and eleven percent of the total weight of -all the oxides, and corresponding to an amount therebetween giving substantially maximum cathode life within sad range.

15. A iluorescent lamp comprising a sealed light-transmitting envelope, a coating of iluorescent material on the inner wall of said envelope, a quantity of mercury in said envelope, a filling of inert gas in said envelope, illamentary electrodes in said envelope, and a coating on said electrodes, said coating consisting essentially of oxides and including at least one oxide of an alkaline earth metal and a quantity of zirconium dioxide in a critical amount, between zero percent and eleven percent of the total weight of all the oxides, and corresponding to an amount therebetween giving substantially maximum cathode life within said range.

16. A uorescent lamp comprising a sealed light-transmitting envelope, a coating of iluorescent material on the inner wall of said envelope, a quantity of mercury in said envelope, a filling of inert gas in said envelope, a iliamentary electrode of coiled tungsten wire at each end of said envelope, and an electron-emissive material carried by said electrode and consisting essentially of oxides and including at least one oxide of an alkaline earth metal and a quantity of zirconium dioxide in a critical amount, between zero percent and eleven percent of the total weight of all the oxides, and corresponding to an amount therebetween giving substantially maximum cathode life within said range.

17. A iluorescent lamp comprising a sealed elongated light-transmitting tubular envelope, a coating of iluorescent material on the inner wall of said envelope, a quantity of mercury in said envelope, a filling of inert gas at low pressure within said envelope, a iilamentary electrode at each end of said envelope and wound into a minor coil with the latter wound into a major coil to form a. coiled-coil, and aneleetron-emissive oosting material filling the space within said minor coil only and" comprising at least-one oxide of an alkaline-earth metal and a. quantity of zireonium dioxide. in a critical amount between zero and eleven percent of the total weight of all the oxides and corresponding to an amount therebetween giving substantially maximum cathode life within said range.

18. The combination of claim 14, in which the koxide of the alkaline earth metal is a reduced carbonate.

19. The combination of claim 13, in which the iilamentary electrode of coiled tungsten wire is wound into a minor coil-.with the latter wound intoa major coil to form a coi1edcoi1,-and the electron emissive material iills the space within said minor coil only.

20. The combination of claim 13, in which the oxide of the alkaline earth metal is a reduced carbonate.

ERWIN F. LOWRY. AERIC L. MAGER.

GERALD W. KEILI-ILTZ.`

REFERENCES CITED The following references are of record in the le oi this patent:

UNITED S'VIATES PATENTS Spanner July 15,1941 

